1. Field of the Invention
The present system and method pertains to communication systems and methods of the types used in communication networks and distribution services. More particularly the present invention relates to radio frequency (RF) transmission systems having modular architectures that accept an input signal(s), amplifies the signal(s) using modular components, and transmits the amplified signal(s) via radio waves to at least one receiver.
2. Discussion of the Background
By their very nature wireless technologies do not require the same investment in infrastructure that originally motivated the United States (U.S.) to permit local telephone companies and cable operators to create "wired" service monopolies. However, in order to compete with "wired" technologies, wireless technologies have a number of unique obstacles that must be overcome. These obstacles include distributing radio waves to areas located in valleys, blocked by mountains, buildings, etc. as well as maintaining a network of high powered transmitters used to reliably broadcast the radio waves to service areas.
Multi-channel multi-point distribution service (MMDS) is generally referred to as "wireless cable," and is one of the wireless service industries that faces the inherent obstacles associated with wireless technologies. Although this document refers to MMDS and wireless cable synonymously, the following other channels of wireless cable when appropriate will be described individually: multi-point distribution service (MDS); Instructional Television Fixed Service (ITFS); and private operational fixed service (OFS).
One of the obstacles facing MMDS service providers is "blocking". In order to further illustrate blocking, FIG. 1 shows a conventional MMDS system where blocking is not a problem. A main transmitter 1 is capable of directly transmitting an analog signal to a receiver 2 without any intervening structures blocking the transmitted signal. When blocking does not occur, the receiver 2 couples the transmitted signal through an antenna 3 and to a home-unit 4. Once in the home-unit, a receiver 5 receives the signal and passes it to a set top 6 which descrambles and tunes the signal. A television 7 then displays a video image that was carried by the signal.
FIG. 2 illustrates a scenario where blocking is a problem. A MMDS signal 8 is blocked by an obstruction (e.g., a hill) 9 so that the receiver 2 cannot receive the MMDS signal. It is estimated that blocking reduces a service area of an MMDS service provider from a theoretical value of 100% to 40%.
FIG. 3 illustrates a conventional approach developed by the MMDS industry to counter the effects of blocking and improve the service area. A booster 10 is strategically placed such that it can receive the MMDS signal from the main transmitter 1 and rebroadcast the MMDS signal to the receiver 2. Thus, the booster 10 can improve service area coverage because the booster 10 can cover areas that are outside of a line-of-sight of the main transmitter 1.
There are generally two types of MMDS boosters 10 used to improve service area coverage. A first type of MMDS booster is a single channel booster which receives 1 of 33 different MMDS channels (actually carriers which are capable of holding multiple audio/video programs) broadcast from the main transmitter 1, and amplifies and transmits that selected single channel to a receiver 2. Because the single channel boosters only process one channel, they do not provide service for the remaining 32 channels. A second type of MMDS booster is a broadband booster, which amplifies all of the channels (typically 33) transmitted from the main transmitter 1 and rebroadcasts the channels to the receiver 2. The broadband boosters require power amplifiers having greater power than those of the single channel boosters because the broadband boosters must amplify up to 33 MMDS channels while single channel boosters amplify only one channel.
As a practical matter, boosters 10 (single channel and broadband boosters) are expensive because they contain a single-unit power amplifier as well as supporting electronics. The expense of the boosters 10 is further escalated by the operational need to have two power amplifiers, or more often, a back-up booster system 10 in order to improve booster reliability. When only redundant power amplifiers are used, one amplifier serves as the operational amplifier and the other is used as a spare. However, aside from the high expense, the use of two single-unit amplifiers is not optimum because if one of the amplifiers fails, service to the service area will cease until the back-up amplifier can be brought on-line. Similarly, when two booster systems 10 are used to improve reliability, the back-up booster system 10 will not begin to broadcast until the operational booster system 10 fails. A gap in service to the service area will exist during the period when the operational booster system 10 fails and the back-up booster system 10 is brought on-line.
Co-channel interference is a second obstacle facing the MMDS industry and limits the effectiveness of current MMDS boosters. Co-channel interference occurs when a stray signal (say from a neighboring one of the boosters 10) interferes with an intended signal by acting as a coherent noise source. Co-channel interference has been particularly problematic with analog MMDS signals, preventing the boosters 10 from being placed close to one another for fear that their respective signals would cause co-channel interference. However, with digital MMDS signals, digital signal processing techniques have been developed that effectively combat co-channel interference so more boosters 10 may be used to cover greater percentages of the MMDS service area.
Even though more of the MMDS boosters 10 may be used in a given service area, the single-unit amplifier architecture of the MMDS boosters 10 (single channel and broadband boosters) is problematic in that when the single unit amplifier fails, service from that MMDS booster 10 is interrupted until a spare amplifier can be brought on-line. Similarly, when a back-up booster 10 is used, service is interrupted until the back-up booster 10 is brought on line. Thus, the MMDS boosters 10 do not degrade gracefully, but rather, fail with little notice, making the MMDS boosters 10 difficult to maintain. When individual ones of the boosters 10 fail, other neighboring boosters are not equipped to increase their transmission powers in order to compensate for the failed booster 10. While some MMDS boosters 10 are equipped to communicate with a network manager, the communication is generally "reactive", in that the failed booster 10 reports its failure, but is incapable of reconfiguring itself in order to restore service.
FIG. 4 illustrates an exemplary conventional broadband MMDS booster 10 which may or may not be supported by a back-up booster 10 (not shown). MMDS signals received through a receive antenna 11 are passed through a low noise amplifier 12, where they amplified. From the LNA 12, the RF signals are passed through a power amplifier assembly 13, which includes a single-unit amplifier, where the RF signals are amplified and passed to a conventional antenna coupler 17. The antenna coupler 17 couples the output signal from the PA assembly 13 and passes the signal to the transmit antenna 18. The amplifier unit 14 is typically powered by a system power supply 15 that distributes low voltage direct current (DC) voltage to all of the components of the booster 10. A control head 16, which is placed in a separate housing than either the power supply 15, the PA assembly 13, or a coupler 17, provides system control functions such as turning on and off the PA assembly.
The amplifier unit 14 is a single-unit amplifier and is not simultaneously operated with one or more of the power-amplifiers for several reasons. First, conventional combining networks for combining output powers from multiple power amplifiers may become damaged when one of the amplifier units 14 fails and in any event would result in several dB of lost output power when one amplifier fails. Second, combining losses are appreciable, and it is less expensive to increase broadcast power by using a larger single-unit amplifier. Third, MMDS booster operators would have to take the second amplifier off-line before removing the failed amplifier unit 14 because conventional MMDS amplifiers are not "hot-swappable". Fourth, maintaining constant output power from each of the single-unit amplifier assemblies 14 would be necessary in order to avoid significant combining and matching problems in a combiner, but conventional single-unit amplifiers are not well suited to provide constant, matched output power.
Because of all the economic, reliability, and performance limitations on the conventional booster 10, the conventional boosters serve a more limited role in MMDS networks than desired by MMDS service operators. Furthermore, due to their specialized architectures, conventional boosters 10 are not used in other applications such as a main MMDS transmitter (single channel or multi-channel), a Personal Communications Services (PCS) base station, or a general purpose reconfigurable transmitter.